1. Field of the Invention
The invention relates to an image processing method. The invention concerns notably an image processing method for producing one or more processed images in which changes in an object to be imaged are clearly reproduced. An image processing method of this kind is used particularly in angiography. During angiography images are formed of the blood vessels of a patient to be examined. To this end, a contrast agent is applied to the vascular system of the patient to be examined. First the arteries in the part of the body of the patient to be examined are filled with the contrast agent which later also reaches the veins. The type of contrast agent may differ, in dependence on the technique used to image the blood vessels; however, the contrast agent ensures in any case that the blood vessels are suitably detected by means of the relevant technique. The blood vessels of the patient to be examined can be imaged by means of magnetic resonance techniques utilizing a contrast agent in the form of a substance which produces a magnetic resonance signal which is stronger than that produced by the surrounding tissue. Such a technique for magnetic resonance imaging of the blood vessels of the patient to be examined is also referred to as magnetic resonance (MR) angiography. The blood vessels of the patient to be examined can also be imaged in X-ray images, the contrast agent then having an X-ray absorption which is higher than that of the surrounding tissue.
2. Description of Related Art
The article Time-resolved contrast enhanced 3D MR Angiography by F. R. Korosec et al in MRM 36 (1996), pp. 345-351, describes a specific MR angiography method.
According to the known MR angiography method, successive magnetic resonance images are formed of a part of the vascular system of the patient to be examined. The magnetic resonance (MR) signals originating from the contents of the blood vessels are acquired during the passage of the blood with the contrast agent through the blood vessels. MR signals are acquired notably while the arteries in the relevant part of the vascular system have already been filled with the contrast agent but the contrast agent has not yet reached the veins. An arterial magnetic resonance image which shows the arteries filled with the contrast agent is derived from such MR signals. After some time, when the contrast agent has also reached-the veins and has not yet disappeared from the arteries, MR signals are acquired again. A venous magnetic resonance image which shows only the veins filled with the contrast agent is derived from the latter MR signals. Subsequently, a subtraction image is formed by subtracting the venous magnetic resonance image from the arterial magnetic resonance image. The subtraction image does offer useful information as regards the passage of blood with the contrast agent through the vascular system of the patient. However, the known MR angiography method cannot be used very well when a more recent, so-called xe2x80x9cblood-poolxe2x80x9d contrast agent is used. Such a contemporary contrast agent remains in the vascular system for a comparatively long period of time, for example from 10 seconds to as long as one minute, so that it takes a long time for the contrast agent to disappear from the arteries. More specifically, such a contemporary contrast agent remains in the vascular system so long that, when the contrast medium reaches the veins, a considerable concentration of contrast agent is still present in the artery whereto the contrast agent has been administered. When the known MR angiography method is used in conjunction with a contemporary contrast agent, therefore, it is necessary to postpone the acquisition of the MR signals for the venous magnetic resonance image for a long period of time after the administration of the contrast agent.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant""s invention of the invention subsequently claimed.
It is an object of the invention to provide a method for imaging a dynamic process, such as the passage of blood with the contrast agent through the vascular system of the patient to be examined, more accurately and faster than the known MR angiography method. It is notably an object of the invention to provide an MR angiography method in which the passage of blood through the vascular system requires less time than in the known MR angiography method.
This object is achieved by means of an image processing method according to the invention wherein
a succession of two or more low-resolution images is acquired,
a series of one or more successive mask images is derived from the low resolution images,
a high-resolution image is acquired,
at least one of the mask images is applied to the high-resolution image as a bandpass filter in order to derive a filtered high-resolution image from the high-resolution image.
The one or more low-resolution images contain image information with a comparatively low spatial resolution. This means that the smallest detail that is faithfully reproduced in the low-resolution images is comparatively large. For example, the low-resolution images contain 64xc3x9764 or 128xc3x97128 pixels for a field of view having a diameter of from 20 to 40 cm. For the same field of view the high-resolution image contains 512xc3x97512 or even 1024xc3x971024 pixels. The low-resolution images are successively acquired, so that the image information in the successive low-resolution images differs as a function of the changes occurring in the imaged object during the dynamic process, for example the progress of the contrast agent in the blood vessels of the patient to be examined.
The one or more mask images relate to variations of the spatial low-resolution image information of one or more successive phases of a dynamic process being studied by means of the image processing method according to the invention. For example, the successive mask images represent respective differences between successive low-resolution images. It is alternatively possible to derive the individual mask images from the low-resolution images in such a manner that such a mask image represents a cumulation of successive differences. For successive mask images the cumulation has then always been continued up to and including another phase of the dynamic process. Such a dynamic process is notably the passage of blood with the contrast agent through the vascular system of the patient to be examined. The phases of the dynamic process concern parts of the dynamic process which take place within finite time intervals. Such a time interval may be short or long, depending on the rate at which changes occur during the dynamic process. For example, in the case of the passage of blood through the vascular system of the patient the individual phases have a duration of approximately 10 seconds. The filling of the arteries with the contrast agent notably takes approximately from 10 to 40 seconds. The high-resolution image relates to the same (part of the) object as the low-resolution images, but the high-resolution image contains inter alia more image information with a much higher spatial resolution. This means that the high-resolution image faithfully reproduces details which are much smaller than the smallest details faithfully reproduced in the low-resolution images. For example, the spatial resolution of the low-resolution images is from five to ten times less than the spatial resolution of the high-resolution images. The one or more mask images are applied to the high-resolution image as a bandpass filter in order to select image information from the high-resolution image on the basis of the brightness values in the relevant mask image. For example, such a mask image is used as a bandpass filter by selecting from the high-resolution image those parts for which the relevant mask image has brightness values in a predetermined range in positions corresponding to positions in those parts of the high-resolution image. For example, it is thus possible to select from the high-resolution image the high-resolution image information which corresponds to a part reproduced with a low resolution in the relevant mask image. The filtered high-resolution image represents image information relating to the relevant phase of the dynamic process, such as the passage of blood with the contrast agent through the vascular system. The high-resolution image is acquired preferably while the contrast agent is present in practically the entire part to be examined of the vascular system of the patient. The filtered high-resolution image contains image information relating to brightness variations over small distances, so that the high-resolution image contains image information relating to small details in the relevant phase of the dynamic process. The relevant phase in the filtered high-resolution image corresponds to the phase of the dynamic process associated with the mask image used. The filtered high-resolution image relates to small details of the relevant phase of the dynamic process. The low-resolution images can be acquired in a fast manner in the course of the dynamic process. For example, low-resolution magnetic resonance images are acquired while the contrast agent flows through the vascular system (arteries and veins) together with the blood. The low-resolution magnetic resonance images are reconstructed from magnetic resonance (MR) signals relating mainly to spatial low-frequency information. This spatial low-frequency information is acquired by scanning only a central part of the reciprocal space of wave vectors (also referred to as k space) in a vicinity of the origin (k=0) during the acquisition of the MR signals. This means that MR signals having a wave vector lying in the central part of the k space are notably used to form the magnetic resonance image. Consequently, the MR signals can be acquired and the low-resolution images reconstructed in a short period of time of, for example one or a few seconds.
Preferably, a plurality of, for example five, ten or fifteen successive mask images are used so as to be applied to the high-resolution image as respective bandpass filters. This yields a number of successive filtered high-resolution images. The progress of the dynamic process can be accurately followed on the basis of such successive high-resolution images. As more low-resolution images are acquired per unit of time, more filtered high-resolution images can be formed per unit of time. The dynamic process can be followed on the basis of such filtered high-resolution images, notably the passage of blood with the contrast agent through the vascular system, the temporal resolution being higher as more low-resolution images are formed per unit of time. The successive mask images which are applied to the high-resolution image in order to obtain the filtered high-resolution image are derived from the respective low-resolution images. According to the invention only one high-resolution image is required. However, it is also possible to use a plurality of high-resolution images in order to derive the filtered high-resolution images therefrom; in any case, the number of high-resolution images required is much smaller, for example some tens of times smaller, than the number of successive low-resolution images. The dynamic process to be examined often progresses so fast that not enough time is available for the acquisition of high-resolution images during the individual phases. According to the invention, however, low-resolution images can be acquired during the individual phases the filtered high-resolution images can be derived therefrom in conjunction with a high-resolution image acquired at a later stage. It is thus achieved according to the invention that the dynamic process can be followed with a high temporal resolution and a high spatial resolution. This means that phenomena in the dynamic process which are of short duration only and take place in a small volume can nevertheless be accurately followed on the basis of the filtered high-resolution images. For example, the high-resolution images clearly illustrate in great detail how small blood vessels are filled with blood with the contrast agent.
The filtered high-resolution image is formed preferably by selecting from the high-resolution image those positions and their pixel values where the pixel value in the corresponding position in the mask image exceeds a predetermined threshold value. This implementation can be advantageously used notably when the difference images between successive low-resolution images are used as the mask images. The pixel values of such difference images constitute the differences between the pixel values of corresponding positions in the successive low-resolution images. Such difference images can be readily derived digitally from the low-resolution images. Moreover, the mask images in the form of the difference images can be readily used digitally as the bandpass filters applied to the high-resolution image.
When the difference between pixel values in the successive resolution images is less than a predetermined ceiling value, usually non-monotonic progress of the dynamic process is concerned. This occurs notably when in MR angiograpy the concentration of the contrast agent in a blood vessel first increases and later decreases again. In positions in the mask images in which the difference between the pixel values in corresponding positions of the low-resolution images is below the ceiling value, pixel values of a predetermined magnitude below the threshold value are added. For example, the ceiling value and the threshold value are equal and preferably have the value zero. The value zero represents, for example a brightness value which is less than all brightness values in the high-resolution image which relate to image information of the object imaged. As a result of this choice, the brightness value is taken to be equal to zero in positions in the individual filtered high-resolution image if the difference between the brightness values in the corresponding positions in the successive low-resolution images is negative. It is thus ensured that in the separate filtered high-resolution images there is selected only image information which relates to a monotonicly progressing part of the dynamic process. For example, only the increase of the concentration of the contrast agent can thus be reproduced in the filtered high-resolution images. It is thus achieved that only the filling of the blood vessels is reproduced in the filtered high-resolution images and not the disappearance of the contrast agent from the blood vessels. However, it is alternatively possible to acquire the low-resolution images during the disappearance of the contrast agent from the veins, thus forming high-resolution images which accurately show the contrast agent leaving the veins.
The invention also relates to a magnetic resonance imaging system and to an X-ray examination apparatus. It is also an object of the invention to provide a magnetic resonance imaging system or an X-ray examination apparatus which is suitable for carrying out the image processing method according to the invention. To this end, the magnetic resonance imaging system and/or the X-ray examination apparatus according to the invention is provided with a signal processing unit arranged to perform the methods of this invention. Such a signal processing unit includes, for example a computer which is programmed to execute the functions of the signal processing unit. It is also possible to provide the signal processing unit with a special purpose (micro) processor which is provided with electronic (integrated circuits) which are arranged to execute the functions of the signal processing unit.
The invention further relates to a workstation arranged to receive a series of low-resolution images and a high-resolution image. For instance, such images may be supplied to the workstation in the form of digital image signals, such as digital electronic videosignals. According to the invention the workstation is arranged to
derive the series of one or more successive mask images from the low-resolution images,
apply at least one of the mask images to the high-resolution image as a bandpass filter in order to derive a filtered high-resolution image from the high-resolution image.
Advantageously, the workstation according to the invention comprises a computer into which a computer program is loaded, which computer program comprises instructions for employing the image processing method according to the invention.
The invention also relates to a computer program comprising instructions for employing the image processing method according to the invention.
These and other aspects of the invention are apparent from and will be elucidated, by way of example, with reference to the embodiments described hereinafter and on the basis of the accompanying drawing; therein: